In the application of coating materials to objects carried upon conveyors through coating stations, variations in the speed of the conveyor will affect the uniformity of the applied coating unless the rate at which the coating is dispensed onto the objects to be coated varies precisely in relation to the speed of the conveyor. The problem of controlling the dispensing rate of the coating material where the conveyor speed tends to vary has been approached by proportionately varying control signals to the dispensing apparatus in relation to the conveyor speed. This approach has been satisfactory where the dispensing apparatus responds linearly to such signals as is often the case in many applications. For example, in the hot-melt adhesive dispensing system described in the commonly assigned U.S. Pat. No. 4,431,690, such a control system is employed. U.S. Pat. No. 4,431,690 is hereby expressly incorporated herein by reference.
In applications such as powder coating systems, air driven venturi powder pumps which supply the powder coating material may not respond linearly to the control air pressure or other control input to the pumps. In addition, different pumps may have unique response characteristics due to differences in internal geometries and structures and in the particular powder coating materials being applied. As a consequence, changes in conveyor speed cannot be accommodated by proportionately changing the control air pressure, or other control signal or input, in direct response to the conveyor speed. For example, in processes for applying super-absorbent powders to diapers in a diaper manufacturing line, the characteristics of the powder coating materials and the powder pumps for transporting the powder coating materials usually result in a response to control inputs or signals by the powder coating material transporting and dispensing apparatus which is not sufficiently linear to achieve a sufficiently uniform powder coating deposition on objects carried by variable speed conveyors.
In many powder spray applications such as the diaper coating example discussed above, as the speed of the conveyor which carries the objects to be coated varies, as it will when increasing to full operating speed upon startup, the controller responds by linearly varying a control signal, often in the form of control air pressure, to a powder pump. The output of the powder pump, however, has not, in prior art powder coating systems, responded in a linear relation to the control signal, and not, consequently, in linear relation to the conveyor speed. The result has been that the application of the coating has not been uniform in relation to the speed of the conveyor.
Thus, there exists a need for a powder coating system control which will accommodate variations in conveyor speed to maintain coating uniformity in coating systems in which powder pump or coating material characteristics result in a failure of the system to respond proportionately to signals which are directly related to conveyor speed.